Resistance spot welding, mainly direct spot welding, has conventionally been used to weld automobile components, starting with automobile bodies. Recently, however, series spot welding, indirect spot welding, and the like have been used.
The characteristics of these three types of spot welding are described below with reference to FIGS. 1(a) to 1(c).
These forms of spot welding are alike in that at least two overlapping metal sheets are welded.
FIG. 1(a) illustrates the direct spot welding method. As illustrated, direct spot welding is a method that squeezes two overlapping metal sheets 1 and 2 from above and below with a pair of electrodes 3 and 4 and applies current while applying pressure with the electrodes 3 and 4, so as to form a spot-like weld 5 using resistance heat generated between the metal sheets. The electrodes 3 and 4 are provided with respective force controllers 6 and 7 and a current controller 8, which constitute a mechanism that allows for control of the pressing force and the current.
The series spot welding method illustrated in FIG. 1(b) is a method of applying current while applying pressure with a pair of electrodes 13 and 14 to two overlapping metal sheets 11 and 12 from the same side (same direction) at separate positions to form spot-like welds 15-1 and 15-2.
The indirect spot welding method illustrated in FIG. 1(c) is a method in which a spot-like weld 25 is formed in two overlapping metal sheets 21 and 22 by holding an electrode 23 against one of the metal sheets 21 while applying pressure with the electrode 23, attaching a feeding point 24 to the other metal sheet 22 at a separate location, and allowing current to flow between the electrode 23 and the feeding point 24.
Therefore, among these three welding methods, the direct spot welding method is used when there is ample space for an opening allowing the metal sheets to be squeezed from above and below. In actual welding, however, there is often not enough room, and it is often difficult to squeeze the metal sheets from above and below in a closed cross-sectional structure. The series spot welding method or the indirect spot welding method is used in such cases.
When the series spot welding method or the indirect spot welding method is used in applications such as those described above, however, the overlapping metal sheets are pressed by an electrode from only one side, with the other side of the metal sheets being unsupported in midair. Therefore, unlike the direct spot welding method that squeezes the metal sheets with electrodes on both sides, it is not possible to apply a high pressing force locally to the weld. Furthermore, since the electrode sinks into the metal sheet during application of current, the condition of contact between the electrode and the metal sheet and between the metal sheets changes over time. For these reasons, the current path between the overlapping metal sheets is unstable, leading to the problem of difficulty in forming a regular fused joint.
To address this problem, JP H11-333569 A (PTL 1) discloses series spot welding in which “in order to form a nugget at a point of contact between overlapping metal sheets, a large current is applied during the initial phase of welding to form an electrode nugget, and then steady current is applied”.
JP 2002-239742 A (PTL 2) discloses that “a seating face that is raised above the surrounding area is formed at the position to be contacted by the electrode, and welding is performed by contacting and applying pressure so as to press against the seating face, yielding a sufficient weld strength without a back electrode”.
As a technique that can also be applied to series spot welding, JP 2006-198676 A (PTL 3) discloses a welding method for indirect spot welding in which “during application of current in series spot welding or indirect spot welding, a time period in which the current is maintained high and a time period in which the current is maintained low are alternately repeated” and also discloses a welding method in which “as the time period in which the current is maintained high and the time period in which the current is maintained low are alternately repeated, the current in the time period in which the current is maintained high is gradually raised”.
As a welding method for forming a regular fused nugget, we developed an indirect spot welding method that adopts two-step control and disclosed this method in JP 2010-194609 A (PTL 4):
“An indirect spot welding method for welding members including at least two overlapping metal sheets by holding a welding electrode against a metal sheet from one side while applying pressure with the welding electrode, attaching a feeding point to the metal sheet at the other side at a location remote from the welding electrode, and allowing current to flow between the welding electrode and the feeding point, wherein with respect to a pressing force of the welding electrode and the current, a time from turning on electricity is divided into a first time period t1 and a second time period t2, and after a pressing force F1 is applied and a current C1 is applied in the first time period t1, a pressing force F2 that is lower than the pressing force F1 is applied and a current C2 that is higher than the current C1 is applied in the second time period t2”.
With respect to the indirect spot welding method disclosed in PTL 4, we also discovered that by limiting the pressing force F1 and current C1 in the first time period t1 and the pressing force F2 and current C2 in the second time period t2 during the two time periods t1 and t2 from the time from turning on electricity, as indicated by Expressions (1) to (4) below, welding can be performed more effectively, and we disclosed this discovery in JP 2012-035278 A (PTL 5):1.2F2≤F1≤5F2  (1)0.25C2≤C1≤0.85C2  (2)35T2.3≤F2≤170T1.9  (3), and2T0.5≤C2≤5.5T0.9  (4)
where T is the total sheet thickness (mm) of the overlapping metal sheets.
As a further development of the above technique, we also progressed from two-step control to three-step control and disclosed this discovery in JP 2011-194459 A (PTL 6):
“An indirect spot welding method for welding members including at least two overlapping metal sheets by holding a welding electrode against a metal sheet from one side while applying pressure with the welding electrode, attaching a feeding point to the metal sheet at the other side at a location remote from the welding electrode, and allowing current to flow between the welding electrode and the feeding point, wherein with respect to a pressing force of the welding electrode and the current, a time from turning on electricity is divided into a first time period t1, a second time period t2, and a third time period t3, a pressing force F1 is applied and a current C1 is applied in the first time period t1, a pressing force F2 that is lower than the pressing force F1 is applied and a current C2 that is higher than the current C1 is applied in the second time period t2, and a pressing force F3 that is the same as or lower than the pressing force F2 is applied and a current C3 that is higher than the current C2 is applied in the third time period t3”.
With respect to the indirect spot welding method disclosed in PTL 6, we also discovered that by limiting the pressing force F1 and current C1 in the first time period t1, the pressing force F2 and current C2 in the second time period t2, and the pressing force F3 and current C3 in the third time period t3 during the three time periods t1, t2, and t3 from the time from turning on electricity, as indicated by Expressions (1) to (6) below, welding can be performed even more effectively, and we disclosed this discovery in JP 2012-091203 A (PTL 7):1.2F2≤F1≤3F2  (1),0.25C2≤C1≤0.9C2  (2),F3≤F2≤3F3  (3),0.5C3≤C2≤0.9C3  (4),30T2.1≤F3≤170T1.9  (5), and2T0.5≤C3≤5.5T0.9  (6)
where T is the total sheet thickness (mm) of the overlapping metal sheets.
We also discovered and disclosed the following in JP 2011-050977 A (PTL 8): “An indirect spot welding method for welding members including at least two overlapping metal sheets by holding a welding electrode against a metal sheet from one side while applying pressure with the welding electrode, attaching a feeding point to the metal sheet at the other side at a location remote from the welding electrode, and allowing current to flow between the welding electrode and the feeding point, wherein by electrically insulating the overlapping surface between the metal sheets, except for a location to be welded, dispersion of current can be suppressed at the time of welding, and a fused nugget can be stably formed”.
Furthermore, we discovered and disclosed the following in JP 2012-157888 A (PTL 9):
“An indirect spot welding method for welding members including at least two overlapping metal sheets by holding a welding electrode against a metal sheet from one side while applying pressure with the welding electrode, attaching a feeding point to the metal sheet at the other side at a location remote from the welding electrode, and allowing current to flow between the welding electrode and the feeding point, wherein welding is performed in a state such that an insulating viscous material is provided between the metal sheets on an entire overlapping surface.
During indirect spot welding for welding members including at least two overlapping metal sheets by holding a welding electrode against a metal sheet from one side while applying pressure with the welding electrode, attaching a feeding point to the metal sheet at the other side at a location remote from the welding electrode, and allowing current to flow between the welding electrode and the feeding point, in particular such that the overlapping metal sheets are pressed by an electrode from only one side, with the other side of the metal sheets being unsupported in midair, then the member exposed to a load by the electrode pressing force warps, and due to the warping the condition of contact between the members changes. Therefore, during indirect spot welding for welding using heat generated by contact resistance, the weld quality is unavoidably affected by warping of the member.
Among conventional indirect spot welding methods, however, a method for obtaining a weld that has fully satisfactory strength even when the rigidity of the members is low has not been disclosed whatsoever.